The use of porcelain facings or veneers (also called porcelain laminates) to cover unsightly teeth and thereby improve their appearance was pioneered by Dr. Charles Pincus in 1928. Dr. Pincus fabricated his porcelain veneers by firing packed dental porcelain powder on platinum foil. All of this early work with porcelain laminates was done using dental porcelain that was fired at 1404.degree. C. (2560.degree. F.).
Because of the limited range of adhesives available at the time, veneers were cemented in place only temporarily. Because of their expense and the limitations imposed by the available adhesives, porcelain veneers were used primarily by movie stars during performances before the camera (for a detailed account of the early history of porcelain veneers see: J. Cosmetic Dentistry, 1 (3), 6-8 (1985).
During the 1970's great improvements were made in the area of dental adhesives, and the use of porcelain veneers became popular among the general public. Because of the limitations in the strengths of existing porcelain, the technique of building a metal substructure and firing porcelain to the outside was also developed. Although this technique was successful and useful, it had its limitations. Paramount among the difficulties associated with porcelain-metal restorations was the need to match the coefficient of thermal expansion of the porcelain and the underlying metal and the need to opacify heavily the porcelain, so that the metal substructure would remain well hidden. The use of porcelain-fused-to-metal construction also made it possible to fabricate more complicated structures, such as porcelain jacket crowns and bridges, but the previously mentioned problems and the difficulty of bonding metal reliably to tooth structure made all-porcelain restorations a desirable goal. (For an extensive review of the application of porcelain to metal see John W. McLean, "The Science and Art of Dental Ceramics--Volume 1: The Nature of Dental Ceramics and Their Clinical Use", Quintessence Publishing Co., Inc., Chicago, 1979).
In order to avoid the need for a metal substructure, much effort has been directed to strengthening dental porcelain. Attempts to strengthen dental porcelain have usually involved the inclusion of strengthening oxide particles in the base porcelain. Examples of strengthening oxides include zirconium oxide (See R. Morena, P. E. Lockwood, A. L. Evans, and C. W. Fairhurst, "Toughening of Dental Porcelain by Tetragonal ZrO.sub.2 Additions", J. Am. Ceram. Soc. 69 (4), C-75-C-77 (1986)) and aluminum oxide (see M. H. Brown and S. E. Sorenson, J. Prosthet. Dent. 42 (5), 507-574 (1979)). The inclusion of strengthening oxides opacifies the porcelain and makes simultaneous control of opacity and strength impossible. The tensile strengths of both traditional dental porcelain as well as metal oxide strengthened porcelain as measured for the present work and taken from various sources in the literature are presented in Table 1.
TABLE 1 ______________________________________ Tensile Strengths of Prior-Art Porcelains Porcelain Tensile Strength (lbs/in.sup.2) ______________________________________ Vitadur N Aluminous Body 5887 .+-. 1294 Vitadur N Aluminous Incisal 5972 .+-. 1294 Trubyte Aluminous Incisal 3626 .+-. 668 Steele's Aluminous Incisal 5972 .+-. 796 Trubyte Aluminous Body 4223 .+-. 355 Steele's Aluminous Incisal 5986 .+-. 882 Vitadur N-Dentine 8200 .+-. 1700 Pentron-Shade Al 8300 .+-. 900 Feldspar Dental Porcelain about 5000 ______________________________________
An ideal porcelain for the fabrication of all-porcelain veneers, crowns and bridges should possess high strength. Ideally, it should possess the strength of the metal-oxide-reinforced porcelains. It should be available in a range of opacities which ideally could run from very opaque to clear. The coefficient of thermal expansion of the porcelain should match the coefficients of thermal expansion of the bonding agents and underlying teeth. It should be available in a variety of shades, and the colorants should be incorporated in, rather than painted on, the porcelain.
Finally, the porcelain should be easy to fabricate by either the platinum foil or refractory model fabrication techniques. It should not show a pronounced tendency to separate during the initial firing, and any separation cracks that do form should heal easily rather than separate further. The maturing temperature should be below 1093.degree. C. (2000.degree. F.) to avoid any unnecessarily severe service for the vacuum furnaces. As a final point, the coefficient of thermal expansion should be less than 15.times.10.sup.-6 .degree.C..sup.-1 in order to avoid difficulty in matching refractory expansion to that of the porcelain.